Information apparatuses such as printers, copying machines, optical media apparatuses, and hard disc apparatuses have been recently required to operate at a higher speed and also downsized. This market environment urges the motors employed in those apparatuses to output more power in a smaller body, and at the same time, the market requires the motors to save power.
Home appliances such as an air-conditioner, refrigerator, hot-water supply, washing machine, employ AC induction motors therein; however those appliances have recently used brushless DC motors instead of the AC induction motors because the brushless DC motors can operate the appliances more efficiently and save power.
In the industrial field, the motor was simply a power source, however, the motor has been recently required to change its speed and operate more efficiently, so that a motor driven by an inverter as well as a brushless DC motor has gained popularity in the industrial field.
In the FA field, a servo-motor is used for driving a robot or a component mounting machine, so that the servo-motor performs accurate driving at variable speeds or accurate positioning.
Those motors employ, in general, the PWM driving method for the power saving and the variable-speed driving. The PWM driving is achieved by the following method: The power transistors coupled to driving coils of the motor are turned on or off, and the ratio of ON vs. OFF is variably set, so that power-feeding to the driving coils can be controlled. This method is well known as a power-thrifty driving method. This PWM driving method has been employed in various motors of home appliances, FA apparatuses and industrial apparatuses. In addition to those fields, information apparatuses have recently started using motors driven by the PWM driving method due to the market environment discussed above.
The motors driven by the PWM driving method employ power transistors suitable for on-off operations, namely, MOSFET or IBGT in general. One feature of those power transistors is that they include gate electrodes insulated with oxide film.
When the power transistor including the gate electrode insulated with oxide film is changed from off-state to on-state, namely, from shut-off state to conductive state, or changed from on-state to off-state, i.e., from conductive state to shut-off state, the following structure is required: In the case of a fast switching speed (a high dV/dt) at forcing the power transistor into conduction or shut-off, the gate driver for driving the gate electrode of the power transistor is equipped with a pulse filter in order to prevent the gate driver from malfunction due to this fast switching speed. (e.g., refer to Japanese Patent Application Non-Examined Publication No. H04-230117).
A structure, where a power transistor is driven by a gate driver, is shown in FIG. 9 as an example of the prior art. FIG. 9 shows a structure of a conventional gate driver.
In FIG. 9, power transistor 802 is a MOSFET and includes a gate electrode insulated with oxide film. In transistor 802, the gate electrode is driven by gate driver 803, so that transistor 802 shifts to the conductive state from the shut-off state, or vice versa. Gate driver 803 includes transistors 831 and 832 which are alternately turned on and off, so that the gate electrode of transistor 802 becomes a plus or zero volt.
In other words, turning-on of transistor 831 and turning-off of transistor 832 force the gate electrode of transistor 802 to take a plus volt and be conductive. Turning-off of transistor 831 and turning-on of transistor 832 force the gate electrode of transistor 802 to take a zero volt and be shut-off.
Gate driver 803 in this structure and with this operation changes power transistor 802 sharply from the shut-off state to the conductive state, or vice versa, because a voltage is quickly applied to the gate electrode of transistor 802 due to the on-off of transistors 831 and 832. This sharp change of transistor 802 increases switching noises and sometimes invites malfunctions in peripheral devices and circuits. The increase of switching noises also sometimes deteriorates transistor 802 per se, and causes malfunctions of gate driver per se.
In order to overcome the problems discussed above, as shown in FIG. 10, resistors 101, 102, diode 103 and capacitor 107 are interposed between gate driver 803 and transistor 802, so that a speed of changing from the shut-off state to the conductive state or vice versa can be adjusted.
Interposing of those elements such as resistors 101, 102, diode 103 moderates the speed of applying a voltage to the gate electrode of transistor 802 due to the input capacitance (not shown) both of those elements and the gate electrode of transistor 802. This mechanism allows adjusting the speed of changing the shut-off state of transistor 802 to the conductive state or vice versa. This technique is disclosed in Japanese Patent Application Non-Examined Publication No. H04-230117.
However, the conventional gate driver discussed above needs a number of elements, such as the foregoing resistors and a diode, to be interposed between the gate driver and the power transistor in order to achieve the following objects: (1) adjusting the speed of changing shut-off state to conductive state or vice versa for lowering the switching noises, (2) operating the power transistor properly in order to prevent the power transistor from being deteriorated.
In the case of forming a motor driving device using the foregoing gate driver, plural power transistors 802a, 802b, 802c, 802d, 802e and 802f are needed to drive motor driving-coils 811, 813 and 815. The elements to be interposed are also needed in the quantity proportionate to the number of those power transistors. To be more specific, resistors 111, 112, 114, 115, 131, 132, 134, 135, 151, 152, 154, 155, diodes 113, 116, 133, 136, 153, 156, and capacitors 117, 118, 137, 138, 157 and 158 are needed.
As such, the conventional gate driver and the motor driving device using this conventional gate driver require a number of elements to be interposed in order to moderate the speed of changing the shut-off state to the conductive state of the power transistors or vice versa. As a result, those elements per se and assembly of those elements boost the cost, and also a layout of the printed circuit board becomes complicated and the area of the printed circuit board increases. Those factors prevent the motor driving device as well as the apparatuses using the device from being inexpensive or downsized.
To overcome the foregoing problems, transistors 831 and 832 forming gate driver 803 can be simply replaced with a constant current source. This replacement can eliminate the interposed elements such as resistors and diodes, and moderate the speed of applying a voltage to the gate electrode of transistor 802 due to the constant current value and the input capacitance of the gate electrode of transistor 802.
However, this simple replacement of transistors 831 and 832 with the constant current source limits power transistors applicable to the gate driver, so that the gate driver cannot be used to a variety of power transistors.
To be more specific, a power transistor has a capacitor structure at its gate electrode which is insulated with oxide film, and the capacitor structure forms an input capacitance. This input capacitance becomes greater as the power transistor has a greater output size, which includes the absolute max. current and withstanding voltage. In other words, an input capacitance of the gate electrode depends on an output size of the power transistor.
Therefore, the foregoing simple replacement of transistors 831 and 832 with a constant current source is applicable only to the power transistors having an input capacitance that matches with the constant current value. In the case of a power transistor having a small input capacitance, the speed of changing conductive state to shut-off state or vice versa becomes too fast, which increases switching noises. On the contrary, a power transistor having a large input capacitance reduces too much the speed of changing conductive state to shut-off state and/or vice versa, which incurs greater switching loss. In other words, only the power transistors having an input capacitance that matches with the constant current value of the constant current source are applicable.
A power transistor including a gate electrode insulated with oxide film has an input capacitance depending on the structure of its gate electrode. In general, a power transistor having a trench structure, which has been recently developed, of its gate electrode has a greater input capacitance than a conventional planer structure. Further, along with the technological progress and the cost reduction of semiconductor, a chip area of a semiconductor having the same capacitance as a conventional one becomes smaller due to finer-chip technology and a shrinking technique, so that the input capacitance also becomes smaller.
The simple replacement of transistors 831 and 832 with a constant current source is thus difficult to support a variety of gate structures of power transistors. In other words, the use of the gate driver in a motor driving device is difficult to support a variety of motor-outputs by changing an output size of a power transistor, and the use thereof is difficult to support power transistors having different structures of gate electrodes.